1. Field of the Invention
This invention relates to a rear conversion lens (teleconverter) mounted on the image side of an objective lens to enlarge the focal length of the objective lens, and particularly to a compact rear conversion lens of good imaging performance mountable also on a telephoto objective lens of great focal length.
2. Related Background Art
An objective lens piece such as a photographic lens will not only become expensive if the focal length thereof becomes e.g. double, but also become bulky and heavy in weight in conformity with the focal length and therefore, it involves not a little difficulty for one to carry a plurality of different objective lenses of relatively great focal lengths with one. In contrast, if use is made of a rear conversion lens (hereinafter abbreviated as "RCL") for enlarging the focal length of an objective lens, the focal length of the objective lens can be enlarged to e.g. double simply by mounting a relatively small and inexpensive additional optical system on the image side of the objective lens, and this is advantageous particularly in respect of portability and cost as compared with a plurality of objective lenses of different focal lengths.
Particularly, in case of a telephoto objective lens, if the focal length thereof becomes nearly double, a single-piece lens will encounter bulkiness and a great increase in cost and therefore, the frequency with which it is used in combination with an RCL is relatively high. Also, recently, bright lenses in which F-number is of the order of 2-2.8 have become used as telephoto objective lenses and it has been desired to make the lenses more compact as a whole. Therefore, the copying with a compact and bright telephoto objective lens has also been desired for an RCL.
Generally, however, an RCL enlarges the focal length of an objective lens on which it is to be mounted and at the same time, enlarges the aberrations of the objective lens, and this leads to the inconvenience that the correction of the aberrations is very difficult. Thus, it has been difficult to provide an RCL which is compact and has a sufficient performance to be practically used.